A directional coupler is a well known four port element for radio frequency equipment. This device allows a sample of a radio or microwave frequency signal, which is provided to an input port and received at an output port, to be extracted from the input signal. Properly designed, the directional coupler can distinguish between a signal provided to the input port and a signal provided to the output port. This characteristic is of particular use in radio frequency transmitter in which both the transmitted signal and a signal reflected from a mismatched antenna can be independently monitored. To obtain such performance, directivity of the coupler should be very high. Directivity of the coupler is high if so called “compensation conditions” are fulfilled. There are two compensation conditions, assuming validity of quasi-static approximation: 1) the capacitive and inductive coupling coefficients are equal, and 2) the coupler is terminated with the proper impedances (preferably 50 Ohms)—for more details see for instance: K. Sachse, A. Sawicki, Quasi-ideal multilayer two- and three-strip directional couplers for monolithic and hybrid MICs, IEEE Trans. MTT, vol. 47, No. 9, September 1999, pp. 1873-1882. Definitions of the coupling coefficients and effective dielectric constants used in the Detailed Description can be found in: K. Sachse, The scattering parameters and directional coupler analysis of characteristically terminated asymmetric coupled transmission lines in an inhomogeneous medium, IEEE Trans. MTT, vol. 38, No. 4, April 1990, pp, 417-425, eq. (2), and the caption of FIG. 7 therein.
Directional couplers intended to be used as monitors of transmitted power or power reflected from an antenna should have weak couplings (coupling of −30 to −40 dB) and high directivity (at least 20 dB). It is a known property of directional couplers that directivity is lower for weakly coupled lines than for tightly coupled ones. Therefore, couplers having a weak coupling are difficult to make so that they are compensated. The article mentioned above by K. Sachse and A. Sawicki describes couplers that are suitable for tight couplings, in the region of −3 dB to −8 dB, corresponding to coupling levels of 0.7 to 0.4. However, weak couplings under compensation conditions can not be obtained with the configurations in the article.
A good solution for these types of couplers is utilizing pure strip line configuration with homogeneous dielectric media. Unfortunately, this solution can be applied only for couplers built as separate components. They can not, or can hardly be applied in an integrated circuit environment where transmission lines carrying a power signal are integrated mainly on the top surface of, or placed beside a multilayer printed board.
Directional couplers formed in coplanar or conductor-backed coplanar and quasi-strip line configurations are described in the U.S. Pat. No. 4,288,760 patent, and here presented in FIG. 1 and FIG. 2, respectively. It can be seen that in both configurations the coupled lines are located at a vertical distance from each other, and also at a horizontal distance from each other. Compensation of these couplers is achievable at only one mutual position of coupled strips, and the corresponding coupling is at a dozen or so dB level. In these couplers, if compensation conditions are to be kept, only a small reduction of coupling is possible by increasing the height of the dielectric layer separating the coupled strips. Moreover, the configuration shown in FIG. 1 is not convenient for multilayer boards, because positions of the external ground planes, e.g. formed by a mechanical construction, are very critical for parameters of the coupler, and small alterations of external ground plane positions will cause large deviations of the coupler parameters.
Directional couplers formed in coaxial line-microstrip printed line configurations are described in the U.S. Pat. No. 5,926,076 and EP 228265 publications. In both configurations the outer conductor of the coaxial line has a longitudinal opening, allowing coupling to a microstrip line etched on a printed circuit board and placed beside the opening. The coupling level can be adjusted in these configurations changing the horizontal distance between the inner conductor of the coaxial line and the microstrip line. However, nothing is mentioned in these publications about whether the couplers are compensated, or how to compensate them.